Controlled deformation of a polymer tube with a restraining surface in fabricating a medical article

ABSTRACT

Methods of manufacturing a medical article that include radial deformation of a polymer tube are disclosed. A medical article, such as an implantable medical device or an inflatable member, may be fabricated from a deformed tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of forming radially expandable medicalarticles through deformation of a polymeric material.

2. Description of the State of the Art

This invention relates to medical article such as expandable medicaldevices used in the treatment of diseased portions of bodily lumen.Expandable medical devices may include certain kinds of implantableendoprosthesis and inflatable members. Inflatable members such asballoons are used, for example, in angioplasty procedures or inimplantation of endoprosthesis which are adapted to be implanted in abodily lumen. An “endoprosthesis” corresponds to an artificial devicethat is placed inside the body. A “lumen” refers to a cavity of atubular organ such as a blood vessel. A stent is an example of such anendoprosthesis. Stents are generally cylindrically shaped devices whichfunction to hold open and sometimes expand a segment of a blood vesselor other anatomical lumen such as urinary tracts and bile ducts. Stentsare often used in the treatment of atherosclerotic stenosis in bloodvessels. “Stenosis” refers to a narrowing or constriction of thediameter of a bodily passage or orifice. In such treatments, stentsreinforce body vessels and prevent restenosis following angioplasty inthe vascular system. “Restenosis” refers to the reoccurrence of stenosisin a blood vessel or heart valve after it has been treated (as byballoon angioplasty or valvuloplasty) with apparent success.

The treatment of a diseased site or lesion with a stent involves bothdelivery and deployment of the stent. “Delivery” refers to introducingand transporting the stent through a bodily lumen to a region, such as alesion, in a vessel that requires treatment. “Deployment” corresponds tothe expanding of the stent within the lumen at the treatment region.Delivery and deployment of a stent are accomplished by positioning thestent about one end of a catheter, inserting the end of the catheterthrough the skin into a bodily lumen, advancing the catheter in thebodily lumen to a desired treatment location, expanding the stent at thetreatment location, and removing the catheter from the lumen. In thecase of a balloon expandable stent, the stent is mounted about a balloondisposed on the catheter. Mounting the stent typically involvescompressing or crimping the stent onto the balloon. The stent is thenexpanded by inflating the balloon. The balloon may then be deflated andthe catheter withdrawn. In the case of a self-expanding stent, the stentmay be secured to the catheter via a retractable sheath or a sock. Whenthe stent is in a desired bodily location, the sheath may be withdrawnwhich allows the stent to self-expand.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel lumen. Therefore, a stent must possess adequateradial strength. Radial strength, which is the ability of a stent toresist radial compressive forces, is due to strength and rigidity alongthe circumferential direction of the stent. Radial strength andrigidity, therefore, may be also be described as, hoop orcircumferential strength and rigidity. Additionally, the stent shouldalso be longitudinally flexible to allow it to be maneuvered through atortuous vascular path and to enable it to conform to a deployment sitethat may not be linear or may be subject to flexure. The material fromwhich the stent is constructed must allow the stent to undergo expansionwhich typically requires substantial deformation of localized portionsof the stent's structure. Once expanded, the stent must maintain itssize and shape throughout its service life despite the various forcesthat may come to bear on it, including the cyclic loading induced by thebeating heart. Finally, the stent must be biocompatible so as not totrigger any adverse vascular responses.

The structure of a stent is typically composed of scaffolding thatincludes a pattern or network of interconnecting structural elements orstruts. The scaffolding can be formed from wires, tubes, or sheets ofmaterial rolled into a cylindrical shape. The scaffolding is designed toallow the stent to be radially expandable. The pattern should bedesigned to maintain the longitudinal flexibility and radial rigidityrequired of the stent. Longitudinal flexibility facilitates delivery ofthe stent and radial rigidity is needed to hold open a bodily lumen.

Stents have been made of many materials such as metals and polymers,including biodegradable polymer materials. A medicated stent may befabricated by coating the surface of either a metallic or polymericscaffolding with a polymeric carrier that includes an active agent ordrug. In many treatment applications, the presence of a stent in a bodymay be necessary for a limited period of time until its intendedfunction of, for example, maintaining vascular patency and/or drugdelivery is accomplished. Therefore, stents fabricated frombiodegradable, bioabsorbable, and/or bioerodable materials such asbioabsorbable polymers may be configured to meet this additionalclinical requirement since they may be designed to completely erodeafter the clinical need for them has ended.

Conventional methods of constructing a stent from a polymer materialinvolve extrusion of a polymer tube based on a single polymer or polymerblend and then laser cutting a pattern into the tube. An advantage ofstents fabricated from polymers is that they can possess greaterflexibility than metal stents. Other potential shortcomings of metalstents include adverse reactions from the body, nonbioerodability, andnon-optimal drug-delivery. However, a disadvantage of polymer stentscompared to metal stents, is that polymer stents typically have lesscircumferential strength and radial rigidity. Inadequate circumferentialstrength potentially contributes to relatively high recoil of polymerstents after implantation into vessels. Another potential problem withpolymer stents is that struts can crack during crimping, especially forbrittle polymers. Furthermore, in order to have adequate mechanicalstrength, polymeric stents may require significantly thicker struts thana metallic stent, which results in an undesirable larger profile.Therefore, methods of manufacturing polymer stents that improvecircumferential strength and radial rigidity are desirable. Theembodiments presented herein address the issue of improvingcircumferential strength and radial rigidity in polymer stents.

As indicated above, inflatable members may include angioplasty and stentdelivery balloons. Angioplasty and stent delivery balloons are typicallymade of polymeric materials. In general, the polymeric material isextruded into tubular shapes or parisons. The extruded parison is thenformed into the balloon shape using a blow molding process. A balloonblow molding process includes a mold, a temperature source, a pressuresource, and a tension source. In the balloon molding process, theextruded tubing is placed inside the mold and subsequently the mold isheated with the temperature source. The tubing may be stretchedlongitudinally under the influence of the tension source and is expandedunder the influence of the pressure source. The pressure sourcetypically consists of a nozzle connected to one end of the parison. Thenozzle is configured to blow air into the parison to expand the parisonwithin the confines of the mold. The final balloon shape is primarilydetermined by the geometric design of the mold and process parameters.

Furthermore, high circumferential strength and modulus are alsoextremely important for inflatable members, such as catheter balloonsfor use in angioplasty procedures and for delivering stents.Additionally, thinner walls are also strongly desirable for inflatablemembers since a low form factor of the balloon facilitates transport ofthe balloon through a vessel. Methods described above that are typicallyused for forming inflatable members do not allow adequate control overcircumferential strength and modulus, as well as wall thickness. Inaddition, such methods are unable to fabricate balloons of a desiredsize out of some materials. Failure of the expanding parison oftenoccurs during fabrication. The shortcomings of inflatable memberfabrication are addressed by embodiments presented herein.

SUMMARY OF THE INVENTION

The present invention includes embodiments of a method for fabricating amedical article including deforming a polymer tube that is at leastpartially immersed in a liquid. The immersed tube may be heated with theliquid. An embodiment of the method may further include fabricating animplantable medical device from the deformed tube or using the deformedtube as an inflatable member.

Another aspect of the invention may include deforming a polymer tube atleast by increasing a pressure inside of the tube with an incompressibleor substantially incompressible fluid. The method may further includefabricating an implantable medical device from the deformed tube orusing the deformed tube as an inflatable member.

In a further aspect of the invention a method for fabricating a medicalarticle may include treating at least a portion of a polymeric tube witha solvent capable of inducing crystallization in the polymer. The methodmay further include deforming the polymer tube. An implantable medicaldevice may then be fabricated from the deformed and treated tube or thedeformed and treated tube may be used as an inflatable member.

In addition, a method for fabricating a medical article may, includedeforming a polymer tube and controlling the deformation of the tubewith a movable surface restraining at least a portion of the deformingtube. The method may then include fabricating an implantable medicaldevice from the deformed tube formed with controlled deformation orusing the deformed tube formed with controlled deformation as aninflatable member.

Furthermore, an additional aspect of the invention may include deforminga polymer tube within an annular mold member having at least threeradially movable restraining members within the annular mold member. Thedeformation of at least a portion of the tube may be controlled with atleast three restraining members configured to restrain the deformingtube. An implantable medical device may then be fabricated from thedeformed tube formed with controlled deformation or the deformed tubeformed with controlled deformation may be used as an inflatable member.

Another aspect of the invention may include allowing a polymer tube todeform in a first stage within a chamber initially defined by a firstrestraining surface of an inner mold member slidably disposed at leastin part within an outer mold member with a second restraining surface.The method may further include allowing the deformed tube to deform in asecond stage within a section of the chamber defined by at least aportion of the second restraining surface after sliding the inner moldmember out of the section of the chamber. An implantable medical devicemay be fabricated from the tube deformed in at least two stages or thetube deformed in at least two stages may be used as an inflatablemember.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a tube.

FIG. 2 depicts a stent.

FIG. 3 depicts an embodiment of an inflatable member.

FIGS. 4A-B depict deformation of a polymer tube.

FIGS. 5A-B depict deformation of a polymer tube.

FIG. 6 depicts deformation of a polymer tube.

FIG. 7A depicts a radial cross-section of a conventional mold.

FIG. 7B depicts a radial cross-section of a lobed mold.

FIGS. 8A-C depict axial cross-sections of a mechanism for deforming apolymer tube.

FIGS. 9A-B depict radial cross-sections of a mechanism for deforming apolymer tube.

FIG. 10A depicts a restraining member.

FIG. 10B depicts a cut-out portion of an annular mold member with aslotted, opening.

FIG. 11 depicts a radial cross-section of an annular mold member withrestraining members.

FIGS. 12A-B depict axial cross-sections of a two stage mold apparatus.

FIG. 13 depicts an axial cross-section of a three stage mold apparatus.

FIGS. 14A-B depict axial cross-sections of a two stage mold for aninflatable member.

FIGS. 15A-C depict axial cross-sections of a three stage mold for aninflatable member.

FIGS. 16A-C depict axial cross-sections of a three stage mold for aninflatable member.

FIGS. 17 and 18 depict optical micrographs of crimped stents.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the following terms anddefinitions apply:

The “glass transition temperature,” T_(g), is the temperature at whichthe amorphous domains of a polymer change from a brittle vitreous stateto a solid deformable or ductile state at atmospheric pressure. In otherwords, the T_(g) corresponds to the temperature where the onset ofsegmental motion in the chains of the polymer occurs. When an amorphousor semicrystalline polymer is exposed to an increasing temperature, thecoefficient of expansion and the heat capacity of the polymer bothincrease as the temperature is raised, indicating increased molecularmotion. As the temperature is raised the actual molecular volume in thesample remains constant, and so a higher coefficient of expansion pointsto an increase in free volume associated with the system and thereforeincreased freedom for the molecules to move. The increasing heatcapacity corresponds to an increase in heat dissipation throughmovement. T_(g) of a given polymer can be dependent on the heating rateand can be influenced by the thermal history of the polymer.Furthermore, the chemical structure of the polymer heavily influencesthe glass transition by affecting mobility.

“Stress” refers to force per unit area, as in the force acting through asmall area within a plane. Stress can be divided into components, normaland parallel to the plane, called normal stress and shear stress,respectively. Tensile stress, for example, is a normal component ofstress applied that leads to expansion (increase in volume and/orlength). In addition, compressive stress is a normal component of stressapplied to materials resulting in their compaction (decrease in volumeand/or length). Stress may result in deformation of a material, whichrefers to change in length and/or volume. “Expansion” or “compression”may be defined as the increase or decrease in length and/or volume of asample of material when the sample is subjected to stress. “Strain”refers to the amount of expansion or compression that occurs in amaterial at a given stress or load. Strain may be expressed as afraction or percentage of the original length, i.e., the change inlength divided by the original length. Strain, therefore, is positivefor expansion and negative for compression.

Furthermore, a property of a material that quantifies a degree ofdeformation with applied stress is the modulus. “Modulus” may be definedas the ratio of a component of stress or force per unit area applied toa material divided by the strain along an axis of applied force thatresults from the applied force. For example, a material has both atensile and a compressive modulus.

The tensile stress on a material may be increased until it reaches a“tensile strength” which refers to the maximum tensile stress which amaterial will withstand prior to fracture. The ultimate tensile strengthis calculated from the maximum load applied during a test divided by theoriginal cross-sectional area. Similarly, “compressive strength” is thecapacity of a material to withstand axially directed pushing forces.When the limit of compressive strength is reached, a material iscrushed.

The term “elastic deformation” refers to deformation of an object inwhich the applied stress is small enough so that the object movestowards its original dimensions or essentially its original dimensionsonce the stress is released. However, an elastically deformed polymermaterial may be inhibited or prevented from returning to an undeformedstate if the material is below the T_(g) of the polymer. Below T_(g),energy barriers may inhibit or prevent molecular movement that allowsdeformation or bulk relaxation. “Elastic limit” refers to the maximumstress that a material will withstand without permanent deformation. Theterm “plastic deformation” refers to permanent deformation that occursin a material under stress after elastic limits have been exceeded.

“Solvent” is defined as a substance capable of dissolving or dispersingone or more other substances or capable of at least partially dissolvingor dispersing the substance(s) to form a uniformly dispersed mixture atthe molecular- or ionic-size level. The solvent should be capable ofdissolving at least 0.1 mg of the polymer in 1 ml of the solvent, andmore narrowly 0.5 mg in 1 ml at ambient temperature and ambientpressure. The “strength” of a solvent refers to the degree to which asolvent may dissolve a polymer. The stronger a solvent, the more polymerthe solvent may dissolve.

Embodiments of methods described herein relate to medical articlesincluding implantable medical devices and inflatable members. Theimplantable medical devices that relate to the embodiments describedherein typically include an underlying scaffolding or substrate. Thesubstrate may have a polymer-based coating that may contain, forexample, an active agent or drug for local administration at a diseasedsite. The active agent can be any substance capable of exerting atherapeutic or prophylactic effect. The underlying substrate that iscoated can be polymeric, metallic, ceramic, or any suitable material.Implantable medical device is intended to include self-expandablestents, balloon-expandable stents, stent-grafts, grafts (e.g., aorticgrafts), artificial heart valves, cerebrospinal fluid shunts, pacemakerelectrodes, and endocardial leads (e.g., FINELINE and ENDOTAK, availablefrom Guidant Corporation, Santa Clara, Calif.). The underlying structureor substrate of the device can be of virtually any design.

Polymers can be biostable, bioabsorbable, biodegradable or bioerodable.Biostable refers to polymers that are not biodegradable. The termsbiodegradable, bioabsorbable, and bioerodable are used interchangeablyand refer to polymers that are capable of being completely degradedand/or eroded when exposed to bodily fluids such as blood and can begradually resorbed, absorbed, and/or eliminated by the body. Theprocesses of breaking down and eventual absorption and elimination ofthe polymer can be caused by, for example, hydrolysis, metabolicprocesses, bulk or surface erosion, and the like. It is understood thatafter the process of degradation, erosion, absorption, and/or resorptionhas been completed, no part of the stent will remain or in the case ofcoating applications on a biostable scaffolding, no polymer will remainon the device. In some embodiments, very negligible traces or residuemay be left behind. For stents made from a biodegradable polymer, thestent is intended to remain in the body for a duration of time until itsintended function of, for example, maintaining vascular patency and/ordrug delivery is accomplished.

The underlying structure or substrate of an implantable medical device,such as a stent can be completely or at least in part be made from abiodegradable polymer or combination of biodegradable polymers, abiostable polymer or combination of biostable polymers, or a combinationof biodegradable and biostable polymers. Additionally, a polymer-basedcoating for a surface of a device can be a biodegradable polymer orcombination of biodegradable polymers, a biostable polymer orcombination of biostable polymers, or a combination of biodegradable andbiostable polymers.

Representative examples of polymers that may be used to fabricate orcoat an implantable medical device include, but are not limited to,poly(N-acetylglucosamine) (Chitin), Chitoson, poly(hydroxyvalerate),poly(lactide-co-glycolide), poly(hydroxybutyrate),poly(hydroxybutyrate-co-valerate), polyorthoester, polyanhydride,poly(glycolic acid), poly(glycolide), poly(L-lactic acid),poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide),poly(caprolactone), poly(trimethylene carbonate), polyester amide,poly(glycolic acid-co-trimethylene carbonate), co-poly(ether-esters)(e.g. PEO/PLA), polyphosphazenes, biomolecules (such as fibrin,fibrinogen, cellulose, starch, collagen and hyaluronic acid),polyurethanes, silicones, polyesters, polyolefins, polyisobutylene andethylene-alphaolefin copolymers, acrylic polymers and copolymers otherthan polyacrylates, vinyl halide polymers and copolymers (such aspolyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether),polyvinylidene halides (such as polyvinylidene chloride),polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such aspolystyrene), polyvinyl esters (such as polyvinyl acetate),acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, andcarboxymethyl cellulose. Another type of polymer based on poly(lacticacid) that can be used includes graft copolymers, and block copolymers,such as AB block-copolymers (“diblock-copolymers”) or ABAblock-copolymers (“triblock-copolymers”), or mixtures thereof.

Additional representative examples of polymers that may be especiallywell suited for use in fabricating or coating an implantable medicaldevice include ethylene vinyl alcohol copolymer (commonly known by thegeneric name EVOH or by the trade name EVAL), poly(butyl methacrylate),poly(vinylidene fluoride-co-hexafluororpropene) (e.g., SOLEF 21508,available from Solvay Solexis PVDF, Thorofare, N.J.), polyvinylidenefluoride (otherwise known as KYNAR, available from ATOFINA Chemicals,Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethyleneglycol.

Inflatable members or balloons may be used in catheters for medical,veterinary or research purposes. For example, inflatable members may beused in catheters for human medical treatment, such as, for example,cardiac catheters used in angioplasty or stent delivery for treatment ofheart or arterial disorders. Inflatable members are typically made frompolymeric materials.

Representative examples of polymeric materials for use in fabricatinginflatable members may include, but are not limited to, polyether-blockco-polyamide polymers, such as Pebax® resins available from AtofinaChemicals, Inc. in Philadelphia, Pa. (e.g., Pebax® grades 63D, 70D,72D); polyamides, such as Nylon available from E.I. du Pont de Nemoursand Company of Wilmington, Del. (e.g., Nylon 12); polyurethanes; andPTFE fluoropolymer resin (e.g., Teflon® available from. E.I. du Pont deNemours and Company of Wilmington, Del.).

Implantable medical devices and inflatable members are typicallysubjected to stress during use, both before and during treatment. “Use”includes, but is not limited to, manufacturing, assembling (e.g.,crimping stent on inflatable member), delivery of stent and inflatablemember and through a bodily lumen to a treatment site, and deployment ofstent at a treatment site. Both a scaffolding and a coating on ascaffolding experience stress that result in strain in the scaffoldingand/or coating. For example, during deployment, the scaffolding of astent can be exposed to stress caused by the radial expansion of thestent body. In addition, the scaffolding and/or coating may be exposedto stress when it is mounted on a catheter from crimping or compressionof the stent.

It is well known by those skilled in the art of polymer technology thatmechanical properties of a polymer may be modified by processes thatalter the molecular structure of the polymer. Polymers in the solidstate may be completely amorphous, partially crystalline, or almostcompletely crystalline. Crystalline regions in a polymer arecharacterized by alignment of polymer chains along the longitudinal orcovalent axis of the polymer chains. An oriented crystalline structuretends to have high strength and high modulus (low elongation withapplied stress) along an axis of alignment of polymer chains. Therefore,it may be desirable to incorporate processes that induce alignment ofpolymer chains along a preferred axis or direction into manufacturingmethods of implantable medical devices and inflatable members.

Furthermore, molecular orientation in a polymer may be induced, andhence mechanical properties modified, by applying stress to the polymer.The degree of polymer chain alignment induced with applied stress maydepend upon the temperature of the polymer. For example, below the glasstransition temperature, T_(g), of a polymer, polymer segments may nothave sufficient energy to move past one another. In general, polymerchain alignment may not be induced without sufficient segmentalmobility.

Above T_(g), polymer chain alignment may be readily induced with appliedstress since rotation of polymer chains, and hence segmental mobility,is possible. Between T_(g) and the melting temperature of the polymer,T_(m), rotational barriers exist, however, the barriers are not greatenough to substantially prevent segmental mobility. As the temperatureof a polymer is increase above T_(g), the energy barriers to rotationdecrease and segmental mobility of polymer chains tend to increase. As aresult, as the temperature increases, polymer chain alignment is moreeasily induced with applied stress.

Moreover, application of stress to a polymer may induce polymer chainalignment, and hence, modify its mechanical properties. In particular,polymer chain alignment is more readily induced between T_(g) and themelting temperature of the polymer, T_(m). Rearrangement of polymerchains may take place when a polymer is stressed in an elastic regionand in a plastic region of the polymer material. A polymer stressedbeyond its elastic limit to a plastic region generally retains itsstressed configuration and corresponding induced polymer chain alignmentwhen stress is removed. The polymer chains may become oriented in thedirection of the applied stress which results in an oriented crystallinestructure. The stressed polymer material may have a higher tensilestrength in the direction of the applied stress.

Furthermore, as indicated above, a plastically deformed material tendsto retain its deformed configuration once the deforming stress isremoved. Stress applied to the material subsequent to the initialdeforming stress tends not to cause further deformation of the materialunless the applied stress is greater than the initial stress level thatcaused the deformation. Therefore, the behavior of a plasticallydeformed material may be more predictable within a range of stress. Thestress range may be less than the initial stress applied that caused theplastic deformation.

Additionally, heating a polymer may facilitate deformation of a polymerunder stress, and hence, modification of the mechanical properties ofthe polymer. A polymer deformed elastically with stress facilitated withheating may retain induced polymer chain alignment by cooling thepolymer before relaxing to or towards an unstrained state.

Various embodiments of methods for manufacturing implantable medicaldevices and inflatable members are described herein. In someembodiments, an implantable medical device or an inflatable member maybe fabricated from a polymer conduit or tube. The tube may becylindrical or substantially cylindrical in shape. For example, FIG. 1depicts a tube 10. Tube 10 is a cylinder with an outside diameter 15 andan inside diameter 20. FIG. 1 also depicts a surface 25 and acylindrical axis 30 of tube 10. When referred to below, unless otherwisespecified, the “diameter” of the tube refers to the outside diameter oftube. In some embodiments, the diameter of the polymer tube prior tofabrication of an implantable medical device may be between about 0.2 mmand about 5.0 mm, or more narrowly between about 1 mm and about 3 mm. Incertain embodiments, the diameters of the tubes for fabricatinginflatable members may be between 0.5 mm and 15 mm, or more narrowlybetween, 1.5 mm to 10 mm.

Additionally, manufacturing of an implantable medical device, such as astent, may include forming a pattern that includes a plurality ofinterconnecting structural elements or struts on a tube. In someembodiments, forming a pattern on a tube may include laser cutting apattern on the tube. Representative examples of lasers that may be usedinclude an excimer, carbon dioxide, and YAG. In other embodiments,chemical etching may be used to form a pattern on the elongated tube. Itis desirable to use a laser cutting technique which minimizes a size ofa heat affected zone. A heat affected zone refers to a region of atarget material affected by the heat of the laser. Heat from the lasermay tend to melt at least a portion of polymer in the heat affectedzone. The molecular orientation induced by applied stress may then bedissipated in the melted portion. The corresponding favorable change inmechanical properties may also be reduced.

FIG. 2 depicts an example of a three-dimensional view of a stent 50.Stent 50 includes a pattern with a number of interconnecting structuralelements or struts 55. The embodiments disclosed herein are not limitedto stents or to the stent pattern illustrated in FIG. 2. The embodimentsare easily applicable to other patterns and other devices. Thevariations in the structure of patterns are virtually unlimited.

In addition, as shown in FIG. 2, the geometry or shape of stent 50varies throughout its structure. A pattern may include portions ofstruts that are straight or relatively straight, an example being asection 60. In addition, patterns may include struts that include curvedor bent portions as in a section 65. The pattern that makes up the stentallows the stent to be radially expandable and longitudinally flexible.Longitudinal flexibility facilitates delivery of the stent. Once a stentis delivered to a desired treatment location, the stent may be radiallyexpanded.

FIG. 3 illustrates a representative embodiment of an inflatable memberor balloon. FIG. 3 illustrates an axial cross-section of a balloon 70.An inflatable member may have a uniform diameter or a variable diameter.As shown in FIG. 3, balloon 70 has sections with different diameters:section 75; distal and proximal sections 80 and 85; and tapered sections90. A surface 95 of section 75 may act to radially expand a stent and/ora lumen when balloon 70 is inflated.

Due to stresses imposed on an implantable medical device or inflatablemember during use, it is important for a device to have adequatestrength both in axial and radial or circumferential directions.Therefore, it may be desirable to fabricate an implantable medicaldevice or inflatable member from a polymer tube with adequate strengthin the axial direction, as shown by an arrow 35 in FIG. 1 and in thecircumferential direction as indicated by an arrow 40. A devicefabricated from a tube with biaxial molecular orientation, orequivalently, a tube with a desired degree of polymer chain alignment inboth the axial and the circumferential directions, may exhibit bettermechanical behavior during use. For example, a stent with a highercircumferential strength and modulus may be less susceptible to crackingduring the crimping process. In addition, increased circumferentialstrength and modulus may allow a decrease in strut width, or generally,a decrease in form factor of a stent. Implantable medical devices, suchas stents, and inflatable members fabricated from tubes with biaxialmolecular orientation may possess mechanical properties similar to orbetter than metal stents with an acceptable wall thickness and strutwidth. Several embodiments of manufacturing implantable medical devicesand inflatable members with biaxial orientation, and hence, with desiredmechanical properties are described herein.

Additionally, conventional fabrication processes of inflatable membersinherently induce circumferential alignment of polymer chains sinceinflatable members are typically formed from a polymer tube by radiallydeforming the tube to a larger diameter. Some of the embodimentspresented herein allow a higher degree of circumferential alignment ofpolymer chains and result in better induced mechanical properties thanconventional methods. Some embodiments also allow a larger degree ofdeformation or to larger diameters without failure of the polymer.

Polymer tubes may be formed by means of various types of methods,including, but not limited to extrusion or injection molding.Alternatively, a polymer tube may be formed from sheets or films thatare rolled and bonded. In extrusion, a polymer melt is conveyed throughan extruder which is then formed into a tube. Extrusion tends to impartlarge forces on the molecules in the axial direction of the tube due toshear forces on the polymer melt. The shear forces arise from forcingthe polymer melt through a die and pulling and forming the polymer meltinto the small dimensions of a tube. As a result, polymer tubes formedby conventional extrusion methods tend to possess a significant degreeof axial polymer chain alignment. However, such conventionally extrudedtubes tend to possess no or substantially no polymer chain alignment inthe circumferential direction.

Since the degree of polymer chain alignment is not the same in the axialand circumferential directions, the mechanical properties may also bedifferent in the two directions. This difference may lead to mechanicalinstability in a fabricated implantable medical device or inflatablemember. To reduce or eliminate this difference and increase thecircumferential strength and modulus, a tube may be radially deformedbetween 20% and 900%, or more narrowly, between 100% and 400%.

Therefore, certain embodiments of manufacturing implantable medicaldevices or inflatable members may include inducing circumferentialpolymer chain alignment in a polymer tube through radial deformation toincrease the circumferential strength and modulus. Circumferentialpolymer chain alignment may be induced in a tube after fabrication of atube by extrusion, for example.

In some embodiments, a polymer tube may be deformed at a temperaturebelow the T_(g) of the polymer. Alternatively, it may be desirable todeform the tube in a temperature range greater than or equal to theT_(g) of the polymer and less than or equal to the T_(m) of the polymer.As indicated above, a polymeric material deforms much more readily dueto, segmental motion of polymer chains above T_(g). Deformation inducespolymer chain alignment that may occur due to the segmental motion ofthe polymer chains. Therefore, heating the polymer tube prior to orcontemporaneously with deformation may facilitate deformationparticularly for polymers with a T_(g) below an ambient temperature.Heating the tube contemporaneously with the deformation may be desirablesince the deformation may occur at a constant or nearly constanttemperature. Therefore, the induced polymer alignment and materialproperties may be constant or nearly constant.

In some embodiments, a polymer tube for fabrication of an implantablemedical device or an inflatable member may be deformed radially byincreasing a pressure in a polymer tube, for example, by conveying afluid into the tube. Tension and/or torque may also be applied to thetube. The tube may be positioned in an annular member or mold. The moldmay act to control the degree of radial deformation of the tube bylimiting the deformation of the outside diameter or surface of the tubeto the inside diameter of the mold. The inside diameter of the mold maycorrespond to a diameter less than or equal to a desired diameter of thepolymer tube.

The polymer tube may also be heated prior to, during, and subsequent tothe deformation. In general, it is desirable for the temperature duringdeformation to be greater than or equal to a glass transitiontemperature of the polymer and less than or equal to a meltingtemperature of the polymer. The polymer tube may be heated by the fluidand/or the mold.

Certain embodiments may include first sealing, blocking, or closing apolymer tube at a distal end. The end may be open in subsequentmanufacturing steps. A fluid, (conventionally an inert gas such as air,nitrogen, oxygen, argon, etc.) may then be conveyed into a proximal endof the polymer tube to increase the pressure in the tube. The pressureof the fluid in the tube may act to deform the tube.

The increased pressure may deform the tube radially and/or axially. Thefluid temperature and pressure may be used to control the degree ofradial deformation by limiting deformation of the inside diameter of thetube as an alternative to or in combination with using the mold. Inaddition, it may be desirable to increase the pressure to less thanabout an ultimate stress of the polymer to inhibit or prevent damage tothe tube. The polymer may be deformed plastically or elastically. Asindicated above, a polymer elongated beyond its yield point tends toretain its expanded configuration, and hence, tends to retain theinduced molecular orientation.

Additionally, the pressure inside the tube and the temperature of thetube may be maintained above ambient levels for a period of time toallow the polymer tube to be heat set. In one embodiment, thetemperature of the deformed tube may be maintained at greater than orequal to the glass transition temperature of the polymer and less thanor equal to the melting temperature of the polymer for a selected periodto time. The selected period of time may be between about one minute andabout two hours, or more narrowly, between about two minutes and aboutten minutes. “Heat setting” refers to allowing aligned polymer chainsform crystalline structure at an elevated temperature. Crystallizationis a time and temperature dependent process, therefore, a period of timemay be necessary to allow polymer chains to adopt crystalline structuresat a given temperature that are stable in a deformed state of apolymeric material. Heat setting may also be facilitated by tension.

Alternatively, pressure inside the tube may be allowed to decrease whilemaintaining the temperature of the tube above an ambient temperatureprior to complete realignment of the polymer chains. In this case, thepolymer tube may be “heat shrunk” which refers to a decrease in thediameter of the tube. The polymer tube may be heat shrunk to a desireddiameter. In either case, the polymer tube may be cooled to below theT_(g) either before or after decreasing the pressure inside of the tube.Cooling the tube helps insure that it maintains the proper shape, size,and length following its formation. Upon cooling the deformed tuberetains the length and shape imposed by an inner surface of the mold.

The degree of deformation or expansion may be quantified by a blow-upratio:

$\frac{{Outside}\mspace{14mu}{Diameter}\mspace{14mu}{of}\mspace{14mu}{Deformed}\mspace{14mu}{Tube}}{{Original}\mspace{14mu}{Inside}\mspace{14mu}{Diameter}\mspace{14mu}{of}\mspace{14mu}{Tube}}$In some embodiments, the blow-up ratio of a polymer tube for use infabricating an implantable medical device may be between 1 and 10, ormore narrowly between 2 and 5. In certain embodiments, the blow-up ratioof a deformed tube for use as an inflatable member may be between 1 and10, or more narrowly between 3 and 6.

FIGS. 4A and 4B depict an embodiment of deforming a polymer tube 100before and after deformation, respectively. In FIG. 4B, a fluid conveyedinto a proximal end 110 as indicated by an arrow 115 increases thepressure inside of tube 100. Tube 100 is radially deformed as indicatedby arrows 120. In some embodiments, a tensile force may also be appliedto tube 100 as indicated by an arrow 125.

FIGS. 5A and 5B further illustrate an embodiment of deforming a polymertube for use in manufacturing an implantable medical device orinflatable member. FIG. 5A depicts an axial cross-section of a polymertube 150 with an outside diameter 155 positioned within an annularmember or mold 160. Annular member 160 may act to limit the radialdeformation of polymer tube 150 to a diameter 165, the inside diameterof annular member 160. Polymer tube 150 may be closed at a distal end170. Distal end 170 may be open in subsequent manufacturing steps. Afluid may be conveyed, as indicated by an arrow 175, into an openproximal end 180 of polymer tube 150.

Polymer tube 150 may be heated by heating the gas to a temperature aboveambient temperature prior to conveying the gas into polymer tube 150.Alternatively, the polymer tube may be heated by heating the exterior ofannular member 160. The increase in pressure inside of polymer tube 150facilitated by an increase in temperature of the polymer tube causeradial deformation of polymer tube 150, as indicated by an arrow 185.FIG. 5B depicts polymer tube 150 in a deformed state with an outsidediameter 190 within annular member 160.

Furthermore, as indicated above, increasing the temperature of a polymerby heating may facilitate deformation of a polymer, and hence, inductionof circumferential polymer chain alignment and modification of themechanical properties of the polymer. In general, it is desirable todeform a polymer above the T_(g) of the polymer since the segmentalmobility necessary for realignment of polymer chains is extremelylimited below the T_(g). As a result of the increased segmentalmobility, the modulus of a polymer decreases with increased temperature,making the polymer easier to deform. Consequently, the amount ofdeformation depends on the temperature of a polymeric material.Therefore, it may be necessary for the increase in temperature of apolymer material to be uniform or relatively uniform to achieve uniformor relatively uniform deformation throughout a volume of a polymermaterial. A more uniform deformation of the tube may also result in amore uniform induced circumferential polymer alignment and inducedmechanical properties. In general, the more uniform the enhancement ofmaterial properties due to deformation, the more mechanically stable thedevice is. In particular, localized regions with unfavorable mechanicalproperties that are susceptible to mechanical failure may be reduced oreliminated by more uniform heating, or by a more uniform increase in thetemperature of the polymeric material.

Certain embodiments of a method of fabricating a medical article withuniform or substantially uniform mechanical properties may includeheating a polymer tube with a liquid while at least partially immersedin the liquid. It may be advantageous to completely immerse the tube inthe liquid to facilitate uniform heating of the tube. The method mayfurther include deforming the tube. The heated and deformed tube maythen be used for fabricating an implantable medical device or used as aninflatable member. In some embodiments, the liquid may include a liquidthat does not significantly degrade or adversely modify the polymericmaterial of the tube during the time frame of the heating and/ordeformation process. For example, the liquid may include water,alcohols, oils (e.g., vegetable oil), or any liquid that that is not asolvent for or that may swell the polymer material.

In some embodiments, the liquid may heat the polymer tube to greaterthan or equal to the T_(g) of the polymer and less than or equal to theT_(m) of the polymer. The tube may be immersed in the liquid for aperiod of time sufficient to heat the volume of the polymer tubeuniformly or substantially uniformly throughout the volume of thepolymeric material of the tube to a desired temperature. The period oftime may be a few seconds to 10 minutes or more narrowly from one tofive minutes. As indicated above, in certain embodiments, the tube maybe heated with the liquid prior to, contemporaneously with, and/orsubsequent to deforming the tube. For instance, it may be advantageousto deform the tube while at least partially immersed in the liquid sothat the tube remains at a substantially uniform and constanttemperature during deformation. In addition, the temperature of thepolymer tube may be maintained to heat set and/or heat shrink thepolymer tube during and subsequent to deformation of the tube.

Additionally, deforming the tube may modify mechanical properties of thetube. For instance, deformation may increase the circumferentialstrength and/or circumferential modulus of the tube. In someembodiments, heating the tube uniformly or substantially uniformly withthe liquid may result in a more uniform deformation and modification ofmechanical properties of the tube than heating the tube using othermethods such as with a gas and/or electrical heating.

FIG. 6 depicts an illustration of uniform heating of a polymer tube 200in a deformation process. Polymer tube 200 is immersed in a liquid 205that is held in a vessel 210. A fluid for increasing the pressure inpolymer tube 200 may be conveyed as indicated by an arrow 215 into ahose 225 which is connected to an end 220 of polymer tube 200.

In addition, exposure of a polymer to a solvent may also facilitateinducing circumferential polymer chain alignment and modification of themechanical properties of a polymer. It has been observed that exposing apolymer to a solvent of the polymer induces crystallization in thepolymer. The phenomenon may be referred to as solvent-inducedcrystallization. Some solvents of a polymer tend to inducecrystallization because the solvent lowers the crystallizationtemperature of the polymer. In particular, absorption of a solvent bythe polymer tends to decrease the T_(g) of the polymer. Below the T_(g)of the polymer no or substantially no crystallization of a polymer tendsto occur. Therefore, as a result of a decrease of the T_(g), thetemperature at which polymer chains may have sufficient mobility toalign and crystallize is decreased. Representative solvents that may beused for solvent treatment of polymer tubes include, but are not limitedto, water, isopropyl alcohol, and ethanol. For example, water ormoisture can be used to increase the crystallization rate of poly(lacticacid) and poly(ethylene terephthalate).

As discussed above, a polymer tube may be heated to facilitatedeformation and induction of circumferential polymer chain alignment.The polymer material is heated to above a temperature at which polymerchain alignment may occur, typically the T_(g) of the polymer.Consequently, solvent-treating a polymer tube may decrease thetemperature increase that may be required to facilitate deformation.Since the possibility of degradation of a polymer increases astemperature increase, solvent-treating a polymer tube may beadvantageous, particularly for polymers with high T_(g)'s.

Representative methods of treating at least a portion of the tube with asolvent may include, but are not limited to, immersing at least aportion of the tube in the solvent, spraying at least a portion of thetube with the solvent, and/or applying the solvent to at least a portionof the tube. In addition, as indicated above and illustrated in FIG. 6,the polymer tube may be immersed in a solvent to heat the polymer tube,in addition to treating the polymer tube to induce crystallization. Inother embodiments, it may be preferable to spray or apply a solvent tothe polymer tube to limit the amount of solvent absorbed by the tubingwhich then limits the degree of crystallization induced in the polymer.

Furthermore, the polymer tube may be treated with the solvent prior to,contemporaneously with, and/or subsequent to deforming the tube. In oneembodiment, the polymer tube may be immersed in the solvent for a periodof time to allow for absorption of the solvent and then deformed afterremoval from the solvent. In some embodiments, the immersion time may bebetween two minutes and ten minutes, or more narrowly between threeminutes and seven minutes. In another embodiment, the polymer tube maybe deformed while immersed in the solvent. It may be desirable to deformthe polymer tube in the solvent to maintain a constant and uniform orsubstantially constant and uniform concentration of solvent in thepolymer tube. Some solvents may be relatively volatile in temperatureranges of deformation, and therefore, may evaporate from a tube removedfrom the solvent bath. The evaporation may result in a concentrationgradient of the solvent in the polymer tube resulting in nonuniformdeformation and material properties in the polymer tube.

In an embodiment, a polymer tube may be deformed after a solvent issprayed and/or applied to a polymer tube. Alternatively, the solvent maybe sprayed on and/or applied to the polymer tube during and/or afterdeforming the polymer tube. Spraying and/or applying the solvent duringand/or after deforming may reduce or prevent a concentration gradient inthe polymer tube. Spraying and/or applying the solvent may be desirablefor solvents that tend to strongly dissolve the polymer tube. Ingeneral, the solvent treatment of the polymer tube should be configuredto not significantly degrade the polymer tube during the time frame ofthe treatment process.

Additionally, the choice of a solvent for solvent treating a polymertube may depend on the degree of induced crystallization that isdesired. As the strength of the solvent increases, the inducedcrystallization increases. However, a solvent that is too strong maydegrade the polymer more than is desired during a time frame of thesolvent treatment. The strength of solvent, the time of treatment, andthe temperature during deformation are related process parameters. Asthe strength of the solvent increases, the time required for treatmentand the temperature required during deformation decrease.

Therefore, some embodiments of a method for fabricating a medicalarticle may include treating at least a portion of a polymer tube with asolvent capable of inducing crystallization of the polymer. The methodmay further include deforming the polymer tube. The heated and deformedtube may then be used for fabricating an implantable medical device orused as an inflatable member.

As discussed above, treating the tube with the solvent decreases acrystallization temperature of the polymer. It follows that in someembodiments, treating the polymer tube with the solvent may thendecrease a temperature at which the polymer chain alignment can occur.This, in term, facilitates the deformation of the polymer tube. Theinduced polymer chain alignment, or crystallization, of the polymer mayadditionally facilitate modifying the mechanical properties of thepolymer tube along with the deformation.

As discussed above, radial deformation of a polymer tube may increasecircumferential polymer chain alignment with a concomitant increase inproperties such as in circumferential strength and modulus. However, thechange of polymer properties due to deformation is generally sensitiveto deformation rate, and hence, the induced strain rate. As a result,the induced circumferential polymer chain alignment and induced changesin properties depend on the overall rate of deformation of the tube orthe overall strain rate induced by an applied stress. For instance, therelative amount of radial and axial deformation and the volumetricuniformity of the deformation depend on the rate of deformation.Deforming a polymer tube by increasing the pressure without adequatecontrol of the rate of deformation tends to result in significant radialand axial deformation as well as volumetrically nonuniform deformation.As a result, higher strain rates applied to a tube tend to impart lesscircumferential deformation, and hence, polymer chain alignment.

Additionally, a higher strain rate may result in failure of a tube at asmaller diameter than with a lower strain rate. Therefore, deforming ata lower strain rate allows deformation to a larger diameter, and hence,greater induced circumferential polymer chain alignment, withoutfailure.

In general, free or unrestrained deformation with a compressible fluidor gas from an original diameter to a desired diameter may result inundesirably high strain rates. In free or unrestrained deformation, thedegree of axial orientation tends to be difficult to control.Furthermore, deformation of a polymer tube with a compressible fluidsuch as an inert gas (air, nitrogen, oxygen, argon, etc.) may not allowcontrol of the relative degree of radial and axial deformation and ofthe volumetric uniformity of deformation.

As indicated above, it is desirable to induce a higher degree of radialdeformation rather than axial deformation. There is generally less of aneed to further induce polymer chain alignment in the axial direction,since, as indicated above, conventionally extruded polymer tubes alreadypossess a significant degree of axial polymer chain alignment. Reducingthe degree of axial deformation may increase the degree of inducedcircumferential strength and modulus for a given amount of deformation.As indicated above, it has been observed that decreasing the rate ofdeformation of a polymer tube increases the degree of radial deformationand decreases the degree of axial deformation. Therefore, controllingthe rate of deformation of a tube to be slower than deformation obtainedfrom, for example, free expansion with a compressible fluid, may providea relatively high radial deformation and relatively low or no axialdeformation. Increasing the degree of radial deformation may alsoincrease the circumferential strength and modulus induced by thedeformation. Additionally, slower deformation reduces the possibility ofsudden expansion in a local area of weakness of a polymer tube andpropagation of expansion from that point.

In some embodiments, the use of an incompressible fluid to deform apolymer tube may allow control over the rate of deformation of the tube.A method for fabricating a medical article may include deforming apolymer tube at least by increasing a pressure inside of the tube withan incompressible or substantially incompressible fluid. The pressuremay be increased by conveying the fluid into the tube. For example, theincompressible fluid may be conveyed into a polymer tube as indicated byarrow 115 in FIG. 4B, arrow 175 in FIG. 5A, and arrow 215 in FIG. 6. Insome embodiments, the polymer tube may be heated with the incompressiblefluid. As mentioned above, heating with a liquid, which isincompressible, may result in more uniform heating of the polymer tubethan heating with a gas. Also, since a liquid has a higher specific heatthan a gas, a polymer tube may be heated faster with a liquid. Themethod may further include fabricating an implantable medical device orusing the deformed polymer tube as an inflatable member.

The incompressible fluid may include any fluid that does notsignificantly degrade or adversely modify the polymeric material of thetube during the time frame of the deformation process. The fluid shouldalso not negatively influence the biocompatibility of the polymer tube.For example, the liquid may include water, alcohols, and other organicliquids.

Some embodiments may include controlling a rate of deformation of thepolymer tube by controlling the fluid conveyed into the tube. Increasingthe pressure with an incompressible fluid allows control of thedeformation of the tube with the rate of fluid addition. In general, thedeformation may not be controlled by the rate of fluid addition when acompressible fluid is used. In the case of a compressible fluid, thedeformation is determined by the circumferential strength of the polymertube material. Therefore, in some embodiments, by using anincompressible fluid, the rate of deformation may be controlled to beslower than a rate of deformation with a compressible fluid.

Consequently, the use of an incompressible fluid allows better controlover the rate of deformation than deforming with a compressible fluid.As indicated above, controlling, in particular, reducing the rate ofdeformation of a polymer tube has several advantages. In one embodiment,the rate of deformation of a polymer tube with an incompressible orsubstantially incompressible fluid may be controlled to result in ahigher ratio of radial to axial deformation than with a compressiblefluid. Similarly, the rate of deformation with an incompressible fluidmay be controlled to reduce or eliminate axial deformation. Therefore,the rate of deformation may be controlled to increase a circumferentialstrength and/or modulus of the tube more than deforming the tube with acompressible fluid. In addition, the rate of deformation with anincompressible or substantially incompressible fluid may be controlledto reduce or prevent sudden localized deformation of the tube.

As indicated above, controlling deformation rate of a polymer tube hasseveral advantages. In particular, decreasing the deformation rate of apolymer tube can result in higher radial deformation with little or noaxial deformation. It follows that a decreased deformation rate canincrease the degree of induced circumferential strength and modulus ofthe polymer tube. In addition, deformation of a uniformly orsubstantially uniformly heated tube facilitates uniform deformation andformation of a deformed tube with uniform material properties.

However, the use of an incompressible fluid or a liquid as a means ofcontrolling deformation and for uniform heating may have somedisadvantages. The exposure of some liquids to a polymer tube duringdeformation may adversely affect the biocompatibility of a fabricatedmedical article. For example, water can promote the growth of pyrogens.It may be necessary to remove the water before further processing of adeformed polymer tube after exposure to water in the deformationprocess. Therefore, it may be desirable to perform a controlleddeformation of a polymer tube in a dry or relatively dry environmentwithout exposure of the tube to a liquid.

Fabrication of inflatable members in lobed molds illustrates theadvantages of a reduced expansion rate in a dry environment. Thedeformation in a lobed mold for fabricating inflatable members occurs instages which results in a lower overall expansion rate than deformationin a conventional or cylindrical mold. FIG. 7A illustrates across-section of a conventional mold member 400 with a circularinner-bore 405. A generally circular polymer tube within bore 405 may beformed on an inner surface 410 of mold member 400. A cross-section of amold suitable for forming a triply-lobed balloon is presented in FIG.7B, which illustrates a cross-section of a mold member 415 with atriply-lobed inner-bore 420 with three elliptical, or pinched-ellipticallobes 423 with surfaces 421.

The first stage of a deformation may result in expansion to a minordiameter 424 of mold member 415. A second stage may result in expansionof a tube conforming to surfaces 421 of triply lobed inner bore 420. Ithas been observed that tubing designed for a circular mold of a givendiameter may be deformed beyond a minor diameter without failure whenformed in a lobed mold that is designed to give a larger diameterrelative to the circular mold. These balloons exhibit a thinner wall anda higher modulus than balloons formed in a conventional mold using freeor unrestrained expansion. A 15 percent increase in blow-up ratio hasbeen observed through the use of lobed molds.

Additionally, when lobed molds are used to blow balloons to the samediameter as a conventional circular cross-section mold using the sametubing, additional tension has to be applied when using the lobed moldin order to get the same wall thickness as that obtained using thecircular mold. This indicates that the reduced expansion rate in thelobed mold also reduces the amount of axial orientation imparted to thetubing during the tubing expansion process.

In general, the deformation rate of a polymer tube is lower in a lobedmold than in a conventional cylindrical mold. The reduced deformationrate results in a balloon with thinner walls, less axial deformation,more circumferential deformation (imparting a higher circumferentialmodulus) than a balloon formed in a conventional circular mold. It isdesirable to obtain similar reduction in deformation rate for a balloonformed in a conventional cylindrical mold.

Some embodiments of a method for fabricating a medical article in a dryenvironment may include controlling the deformation of a tube disposedwithin a cavity with a variable size cross-section. The cavity may bedefined by a radially movable surface configured to restrain at least aportion of the deforming tube. The method may further includefabricating an implantable medical device from the deformed tube orusing the deformed tube as an inflatable member. The deformation processmay be performed without exposing the polymer tube to a liquid, orgenerally to a wet environment.

In some embodiments, the tube may be deformed by increasing a pressurein the tube. The pressure may be increased, as described above, byconveying a fluid into the tube. The fluid may be an inert gas, such asair, oxygen, nitrogen, and/or argon. In other embodiments, the fluid maybe a liquid.

In an embodiment, a polymer tube may be positioned adjacent to theradially movable restraining surfaces. A longitudinal axis of the cavitymay be parallel or substantially parallel to a cylindrical axis of thepolymer tube. The polymer tube may be uniformly heated prior todeformation. In addition, the tube may be heated contemporaneously withand subsequent to the deformation. As the polymer tube is deformed by anincrease in pressure inside the tube, at least a portion of the exteriorsurface of the tube may make contact with at least a portion of theradially movable restraining surface. In some embodiments, all orsubstantially all of an outside surface of the polymer tube may be incontact with the restraining surface. The deformation, in particular thedeformation rate, of the tube may be controlled by outward radialmovement of the restraining surface. The rate of deformation of thepolymer tube may be readily controlled by controlling a rate of movementof the restraining surface.

In one embodiment, the surfaces may control deformation by controllingthe radial movement of the surfaces directly through adjustment of thecavity to a desired size. The change in the size of the cavity may becontinuous or in stages or steps. In other embodiments, the restrainingsurfaces may exert a constant or variable force on the surface of thedeforming tube. The surfaces may move radially outward to allowdeformation when the force is less than the force exerted by thedeforming tube.

In certain embodiments, a rate of deformation of the polymer tube may becontrolled to be slower than a rate of unrestrained or free deformationof the tube. Therefore, the rate of deformation may be controlled toincrease the radial deformation and reduce or eliminate axialdeformation. Additionally, a rate of deformation of the polymer tube maybe controlled to increase a circumferential strength and circumferentialmodulus of the tube more than an unrestrained or free deformation of thetube. Further, as mentioned above, unrestrained deformation of a polymertube with a compressible fluid may lead to sudden localized deformationin the tube. Controlling the expansion with a radially movablerestraining surface may reduce or eliminate such sudden localizeddeformation.

In certain embodiments, the polymer tube may be heated prior to,contemporaneously with, and/or subsequent to deformation with a heatedgas conveyed into the tube. In addition, the polymer tube may be heatedwith the restraining surface. The tube is deformed at a temperaturegreater than or equal to a glass transition temperature of the polymerand less than or equal to a melting temperature of the polymer. In otherembodiments, the tube may be deformed at a temperature below the glasstransition temperature.

In one embodiment, the movable restraining surface forms a cavity inwhich the polymer tube may be disposed. The radial cross-section of thecavity may be varied during deformation by movement of the restrainingsurface. There are various ways of forming a restraining surface forcontrolling deformation of a polymer tube. In one embodiment, arestraining surface may include at least three surfaces of longitudinalwedge members arranged to form a cavity. The wedge members may beconfigured to slide in a manner than varies the size of the cavity, suchas in a sliding wedge crimper. Another example of a movable restrainingsurface may include the inside of an iris mechanism. The wedge membersor iris may be constructed from a variety of heat conductive materials.Representative materials may include, but are not limited to, stainlesssteel, beryllium copper, brass, and aluminum.

FIGS. 8A-C depict axial cross-sections of a mechanism 250 and processfor controlled deformation of a polymer tube. FIG. 8A depicts mechanism250 prior to deformation in a preheating stage, FIG. 8B depictsmechanism 250 during deformation, and FIG. 8C depicts mechanism 250 atthe final diameter of the polymer tube. A polymer tube 255 is positionedwithin insulating blocks 260, iris mechanism 265, and collet 270. Collet270 is activated to clamp a proximal end 275 of polymer tube 255 intoplace. A distal end 280 is sealed by a clamp, or otherwise, blocked.FIGS. 9A-B depict radial cross-sections of iris mechanism 265. Asillustrated in FIGS. 9A and 9B, iris mechanism 265 is made up of slidingwedges 290 that form an opening or cavity 295. Iris mechanism 265 hassix sliding wedges 290 that form or define opening 295. More wedges maybe used to more closely approximate the circular cross-section of atube. The size of opening 295 may be varied by sliding wedges 290, asillustrated by FIG. 9B. Walls 300 of wedges 290 move outward as the sizeof opening 295 increases and act as restraining surfaces when polymertube 255 deforms radially.

During the preheating stage illustrated in FIG. 8A, opening 295 of irismechanism 265 may be closed to a diameter slightly larger than thediameter of polymer tube 255. The inside of polymer tube 255 isinitially at atmospheric pressure. The portion of polymer tube 255inside of iris mechanism 265 is heated and allowed to reach atemperature between the T_(g) and the T_(m) of the polymeric material.Polymer tube 255 may be heated by heating iris mechanism 265. Polymertube 255 may also be heated by conveying a heated gas into polymer tube255. The heated gas may be conveyed into polymer tube 255 prior tosealing distal end 280. The pressure of the conveyed gas may be lowenough that it causes an insignificant amount of deformation of polymertube 255 during the preheating stage.

When polymer tube 255 has reached a desired temperature, the pressure inthe portion of polymer tube 255 is increased by conveying a gas intoproximal end 275. The increase in the pressure radially deforms polymertube 255 to a diameter of opening 295 of iris mechanism 265 so that asurface of polymer tube 255 makes contact with the surface of walls 300of opening 295. Opening 295 of iris mechanism 265 is slowly increased asindicated by the arrows 305 to allow radial deformation of polymer tube255. Movable walls 300 control the deformation rate of polymer tube 255to allow uniform expansion along the length of polymer tube 255.

FIG. 8C depicts mechanism 250 when polymer tube 255 is deformed to adesired diameter. Polymer tube 255 may be cooled to a temperature tobelow the T_(g) of the polymer by cooling wedges 290 of iris mechanism265. Polymer tube 255 may also be cooled by conveying a gas at anambient temperature or refrigerated gas through the inside of polymertube 255. Furthermore, polymer tube 255 may be held at the finaldiameter for a period of time before cooling to allow the polymer toheat set.

Certain embodiments of fabricating medical articles such as implantablemedical devices and inflatable members may also include deforming apolymer tube within an annular mold member with at least three radiallymovable restraining members within the annular mold member. The methodmay further include controlling the deformation of at least a portion ofthe tube with at least three of the restraining members configured torestrain the deforming tube. The annular mold member may be constructedfrom polymeric and/or metallic materials. Representative materials mayinclude, but are not limited to, stainless steel, beryllium copper,brass, and aluminum.

In certain embodiments, a tensile force may be applied to the tube tocontrol a degree of axial orientation imparted to the tube during thedeformation process. A suitable tensile force may be 0.5 Newtons to 20Newtons, or more narrowly, 5 Newtons to 15 Newtons.

In some embodiments, the restraining members may be slidably disposedwithin a wall of the mold member. At least a distal portion of arestraining member may extend outward through an opening in the wall ofthe mold member out of an outside surface of the annular member. Atleast a portion of a center portion may extend through an opening in thewall of the annular member. In some embodiments, the distal portion maybe configured such that it cannot slide into the opening in the wall. Atleast a portion of a proximal portion of the restraining member mayextend radially into at least a portion of the annular member. A surfaceof the proximal portion of the restraining member may be configured tomake contact with a polymer tube deforming within the annular moldmember. The restraining members may also be configured to control thedeformation of the tube by exerting an inward radial force that opposesthe outward radial deformation of the tube. In one embodiment, theinward radial force may be less than the outward radial force exerted bythe deforming tube. The slidably disposed annular members may then slideradially outward. As a result, the rate of deformation of the tube iscontrolled to be slower than a free or unrestrained deformation. In analternative embodiment, a distal portion of at least one restrainingmember may be coupled to an inside surface of the annular mold member.

In one embodiment, the inward radial force exerted by the restrainingmembers may be constant (not vary with radial position of therestraining member), increase as the restraining member slides outward,or decrease as the restraining member slides outward. For example, theforce may be exerted by a spring and/or a band coupled to the distal endof the restraining members. For instance, the force exerted by therestraining member may be configured to increase as the restrainingmember slides outward since the force law of a spring or a bandparabolically increases. A constant or nearly constant force may besimulated through the use of a relatively loose or flexible spring.Alternatively, the force exerted by a stiff spring increases steeply asthe restraining member slides radially outward. Alternatively, thedistal end of the restraining members may be coupled to pressurizedpistons. The pistons may be configured to have any functional dependencebetween force exerted by the restraining member and radial position ofthe restraining member.

In an alternative embodiment, the restraining members may allowdeformation of the tube in a step-wise fashion. The restraining membersmay be configured to slide radially outward in pre-defined radialincrements at a pre-defined rate. The size of the increments and therate of the steps may be controlled manually or automatically. In oneembodiment, the distal end of the restraining member may be configuredto be releasably coupled within a series of slots at various radialdistances.

Further, the restrained portion of the tube that is controlled maycorrespond to at least a portion of an outer surface of a selected axialportion of the tube. The selected axial portion may be a proximalportion, a distal portion, and/or from an axial portion to a distalportion. A selected axial portion of the polymer may be controlled bydisposing restraining members at and/or proximate to the selected axialportion of the annular mold member.

In one embodiment, a restraining member may extend radially into andlongitudinally along at least a portion of a cylindrical axis of themold member. A restraining member may be parallel or substantiallyparallel to the cylindrical axis of the mold member. A cross-section ofthe restraining members may be any of a variety of shapes, for example,rectangular, cylindrical, conical, etc. FIG. 10A illustrates arestraining member 455 having a distal portion 466 and a proximalportion 468. FIG. 10B depicts a cut-out portion 445 of an annular moldmember with an outside surface 446, wall 460, and an inner chamber 465.Portion 445 of the annular mold member has a slotted opening 472 in wall460. Proximal portion 468 may be disposed within opening 472 such thatrestraining member 455 is parallel or substantially parallel to acylindrical axis of the mold member as indicated by an arrow 471.Surfaces 473 of distal portion 466 may make contact with outside surface446 of portion 445 and prevent distal portion 466 from sliding into theopening 472. A width 469 of distal portion 466 is larger than a width474 of opening 472 in wall 460. A surface 477 of proximal portion 468 isconfigured to make contact with an outer surface of a deforming tube.

Furthermore, the restraining members may control and/or restrain thedeformation of the tube in a radially symmetric manner along the sameaxial portion of an annular mold member. For example, three restrainingmembers may be radially spaced equally or approximately equally apart,in this case, 120°. FIG. 11 illustrates an embodiment of an annular moldmember with restraining members. In FIG. 11 a radial cross-section of amold member 450 is shown with restraining members 455. Three restrainingmembers 455 are slidably incorporated within openings 472 in wall 460 assliding inserts. Restraining members 455 have distal portions 466extending out of mold member 450 and proximal portions 468 partiallywithin openings 472 of wall 460 and partially within mold chamber 465.As illustrated in FIG. 11, widths 469 of distal portions 466 are largerthan widths of openings 472 in mold wall 460 to prevent distal portions466 from sliding into openings 472. An unexpanded tube 470 is depictedalong with an expanding tube 475 within mold chamber 465. Surfaces 477of restraining members 455 make contact with the outer surface ofexpanding tube 475 and control the rate of expansion. Restrainingmembers 455 apply an inward radial force that opposes the outward radialforce from expanding tube 475. Since the inward radial force is lessthan the outward radial force, restraining members 455 slide radiallyoutward, as indicated by arrows 480. As shown in FIG. 11, the inwardradial force is due to spring tabs 490 coupled to distal portions 466 ofrestraining members 455.

In some embodiments, a rate of deformation of the polymer tube iscontrolled to be slower than a rate of unrestrained deformation of thetube. A slower rate of deformation may reduce or prevent failure of thetube during deformation. A slower rate of deformation may allowdeformation to a desired diameter without rupturing. Deformation to alarger diameter is preferable due to the larger degree ofcircumferential orientation induced. Consequently, a rate of deformationof the polymer tube may be controlled to increase a circumferentialstrength and circumferential modulus of the tube more than a free orunrestrained deformation of the tube. In addition, a slower rate ofdeformation may reduce or eliminate axial deformation of the tube.

In one embodiment, the restraining members may be configured to controlthe expansion of the tube until the outside surface of the deformed tubecontacts an inner surface of the mold member. Alternatively, when thetube has deformed to a desired degree, the force restraining therestraining members may be reduced or eliminated. In addition, therestraining members may be slid radially outward such that no portion ofthe restraining member is within the annular member.

In some embodiments, after deforming the tube to a desired diameter, theradial cross-section of a deformed tube may be distorted. To restore acylindrical or substantially circular radial-cross-section, pressure andoptionally heat may be applied within the deformed polymer tube. Afterthe desired degree of deformation has been achieved, the deformed tubemay then be allowed to heat set and cool.

Numerous variations may be included in a mold member with restrainingmembers. For example, restraining members may also be included in moldmembers configured to fabricate inflatable members with sections havingdifferent diameters, as illustrated in FIG. 3.

Additional embodiments of fabricating a medical article with controlleddeformation may include deforming a polymer tube in a staged moldapparatus. The method may include allowing a polymer tube to deform in afirst stage within a chamber initially defined by a first restrainingsurface of an inner mold member. The inner mold member may be slidablydisposed at least in part within an outer mold member with a secondrestraining surface initially in contact with an outer surface of theinner mold member. In one embodiment, the deformed tube may be allowedto further deform in a second stage within a section of the chamberdefined by at least a portion of the second restraining surface aftersliding the inner mold member out of the section of the chamber.

Prior to the first stage of deformation, the tube may have a diameterless than the diameter of the chamber. The tube may be deformed, asdescribed above, by increasing the pressure inside of the tube. Inaddition, the tube may be heated. In an embodiment, the tube may bedeformed until at least a portion of the outside surface of the tubeconforms to and is restrained by the first restraining surface. The tubemay be heated by conveying gas into the tube. The tube may also beheated by the inner mold member. In one embodiment, the tube may beheated prior to, during, and after deforming the tube with the appliedpressure. In certain embodiments, the deformed tube may be allowed toheat set after the first stage of deformation prior to deforming thetube in a second stage. The tube may be heat set while still underpressure. In one embodiment, the tube may be allowed to cool between thestages. The application of heat and pressure may be reduced oreliminated for a period of time between the first stage and the secondstage.

In some embodiments, the deformed tube may be deformed further in asecond stage. A tube deformed in a first stage may be allowed to deformwithin a section of the chamber defined by at least a portion of thesecond restraining surface after sliding the inner mold member out ofthe section of the chamber. The section of the chamber defined by thesecond restraining surface may be formed by sliding the inner moldmember out of the section. Pressure and heat applied to the inside ofthe tube may cause further deformation of the tube until it isrestrained by the second restraining surface.

Deforming a tube from a first to a second diameter in at least twostages may have several advantages over deforming the tube in a singleunrestrained deformation to the second diameter. In some embodiments,the overall rate of deformation in at least two stages may be slowerthan a single stage unrestrained deformation of the tube from a firstdiameter to a second diameter. As indicated above, a slower deformationrate may reduce or prevent failure of the tube during deformation.Additionally, deforming the tube in at least two stages increases acircumferential strength and circumferential modulus of the tube morethan a single stage unrestrained deformation of the tube from a firstdiameter to a second diameter. Deforming the tube in at least two stagesalso reduces or eliminates axial deformation of the tube.

Additionally, a method of fabricating an inflatable medical device mayfurther include fabricating adjustable length inflatable members asdisclosed in U.S. Patent Publication No. 20020125617. In addition,inflatable members with sections having different diameters may also befabricated by deformation of a polymer tube in stages.

Other embodiments of the method may include deforming the polymer inmore than two stages. In one embodiment, an additional third stage mayinclude allowing the tube to deform within a second section of thechamber, the second section defined by at least a portion of a thirdrestraining surface after sliding the outer mold member out of thesecond section of the chamber. The outer mold member may be slidablydisposed at least in part within a second outer mold member having thethird restraining surface. In one embodiment, the second section may beat least a part of or greater than the section in which the second stageof the deformation was performed.

FIGS. 12A-B illustrate an embodiment of a two stage deformation of apolymer tube. FIG. 12A depicts an axial cross-section of a two stageblow mold apparatus 500. Mold apparatus 500 includes a proximal moldassembly 505 which includes a mold bore member 510 and a distal moldassembly 515 which includes an outer mold member 520. An inner moldmember 523 is slidably disposed within outer mold member 520 and overmold bore member 510. The first stage of the deformation may beinitiated by forming a chamber 525 with a length 527 and diameter 529.Chamber 525 is initially defined by inner restraining surface 530 ofinner mold member 523. Chamber 525 is formed by sliding mold bore member510 as indicated by an arrow 535. A tube disposed in chamber 525 isdeformed radially in a first stage by applying heat and pressure to thetube. The tube may be deformed until at least a part of an outer surfaceof the tube is restrained by inner restraining surface 530.

As illustrated in FIG. 12B, the second stage of the deformation isperformed by sliding inner bore member 510 and inner mold member 523 asindicated by arrow 540 to increase diameter 529 and length 527 ofchamber 525. In the second stage, chamber 525 is defined by an outerrestraining surface 560 of outer mold member 520. The tube deformed inthe first stage is further deformed by applying heat and pressure. Thedeformation of the tube in the second stage is restrained by secondrestraining surface 560.

FIG. 13 illustrates a mold apparatus 580 configured to perform a threestage deformation of a tube. Mold apparatus 580 includes a first innermold member 585, a second inner mold member 590, and an outer moldmember 595. In a first stage, first inner mold member 585 allowsdeformation of the tube to a diameter 600. In a second stage, the tubeis allowed to deform to a diameter 605 by second inner mold member 590.The tube is then allowed to deform to a diameter 610 by outer moldmember 595 in a third stage.

FIGS. 14A-B depict axial cross-sections of a two-stage inflatable membermold 700. Mold 700 has an inner annular mold member 705 and an outerannular mold member 710. FIG. 14A shows a first stage configuration inwhich a polymer tube can be expanded in chamber 715 to a diameter 720.FIG. 14B shows a second stage configuration in which the polymer tubecan be expanded from diameter 720 to a diameter 725.

FIGS. 15A-C depict axial cross-sections of a three-stage inflatablemember mold 750. FIG. 15A shows a first stage configuration in which apolymer tube can be expanded in a chamber 755 to a diameter 760. FIG.15B shows a second stage configuration in which the polymer tube can beexpanded from diameter 760 to a diameter 765. FIG. 15C shows a thirdstage configuration in which the polymer tube can be expanded fromdiameter 765 to a diameter 770.

Furthermore, a staged mold may be used to fabricate expanded tubes ofthe same length, but with different diameters. FIGS. 16A-C depict axialcross-sections of three configurations of a three-stage inflatablemember mold 800. As shown in FIG. 16A, mold 800 can be used to fabricatean expanded tube with a diameter 805 and a length 810. FIG. 16B showsthat mold 800 can be used to fabricate an expanded tube with a diameter815 and length 810. FIG. 16C shows mold 800 can be used to fabricate anexpanded tube with a diameter 820 length 810.

Some embodiments of staged deformation may include an inner mold memberwith a circular or substantially circular cross-section, similar to thatshown in FIG. 7A, with a first restraining surface that is a cylindricalor substantially cylindrical surface. Certain embodiments may include aninner mold member with a noncircular cross-section along at least aportion of a longitudinal axis of the inner mold member. The variationsin cross-sectional shapes of the inner mold member are virtuallyunlimited. One embodiment may include an inner mold member having atleast one lobe or slot running axially along at least a portion of theinner mold member. An embodiment may include at least one indentationrunning axially along at least a portion of the inner mold member. Someembodiments may include a lobed cross-section, similar to that depictedin FIG. 7B. Another embodiment may include a cross-section with slotsformed between indentations similar to restraining members shown in FIG.11.

In an embodiment, a tube may be deformed in a first stage in an innermold member with a noncircular cross-section. The tube may then bedeformed in a mold with a circular cross-section by sliding the firstmold member out of a section of the chamber.

Example Wet Deformation

Some embodiments of the present invention are illustrated by thefollowing Example. The Example is being given by way of illustrationonly and not by way of limitation. The Example illustrates wetdeformation of a polymer tube. The parameters and data are not to beconstrued to unduly limit the scope of the embodiments of the invention.

The sample was a poly(L-lactic acid) tube with 0.09/0.065 (0.09 inoutside diameter (OD) and 0.065 in inside diameter (ID)). The tube wasdeformed in a 0.136 in glass mold immersed in a circulating water bathat 55° C. One end of the tube was sealed by heating the end of thetubing with an air box at 225° F. and clamping it with pliers when itsoftened. The other end of the tube was connected to a water pump. Theair was pumped out and the water was pumped in. The tubing was placed inthe glass mold. The tube in the mold was immersed in the heated waterbath for 10 minutes. One end of the end of the glass mold was left outof the water. The sealed end of the tubing was kept inside the glassmold

The pressure was slowly increased in the tube after 10 minutes in thewater bath. The tube was pressurized with a 20/30 indeflator. One end ofthe glass mold was held against the bottom of the water tank to keep thetubing from leaving the mold during the initial expansion. The tube wasdeformed until the deformed tubing was at the water line on the mold.The indeflator pressurized the tube to approximately 14 atm.

The tubing was left pressurized in the hot water for 4 minutes. The tubeand mold were then removed from the hot water and placed in a roomtemperature water bath for 1 minute. The pressure was released after oneminute. The tube was removed from the mold. The sealed end of the tubewas cut and air was blown through the tubing to dry it.

The deformed tubing had a 0.006 in wall thickness and an OD of 0.136 in.This gives a blow-up ratio of 2.1 (110% radial expansion) and an areadraw down ratio of 1.21 (only 20% axial elongation). Area draw downratio is defined as the cross-sectional area of tubing divided by thecross-sectional area of expanded tubing.

In addition, samples were prepared by deforming the tubes with a gas.The tubes were deformed as illustrated and described in reference toFIGS. 4A-B. Tubing was loaded onto a N1782 balloon machine using a glassmold. The tubing was connected to a pressure source using a collet andheated through the glass mold using heated forced air. Four groups ofsamples were prepared. Process parameters for the four samples include:

Blow pressure—334 psi

Preheat time—15 sec

Tension—100 gm

Temperature 225 F

Nozzle speed=2 mm/sec (Group 1-3)

Nozzle speed=5 mm/sec (Group 4)

Nozzle speed refers to speed of a heating nozzle that travels along thelength of the mold to heat the mold and tubing. Only a section of thetubing is heated at any given time. Nozzle speed determines the rate ofheating for a given section The slower the nozzle speed, the higher therate of heating for a heated section. Samples 1, 3, and 4 were 100%poly(L-lactic acid) tubes and sample 2 was an 80/20 blend ofpoly(L-lactic acid) and poly(trimethylene carbonate).

A summary of the results of the gas-deformed and water-deformed samplesare shown in Table 1. Samples 1 to 4 correspond to tubes deformed withgas and sample 5 was deformed with liquid. Samples blown using hot airand air pressure were compared with the water expanded samples under abirefringence lamp to examine the stress distribution. The waterexpanded tubing had a more uniform stress distribution indicating a moreuniform deformation.

TABLE 1 Summary of deformation runs. Mold Pre-def. Post-def. Pre-Def.Wall Def. Wall OD OD/ID OD/ID Thickness Thickness Group (in) (in) (in)(in) (in) 1 .094 .066/.039 .0935/.0845 0.0135 .0045 2 .094 .066/.039.0935/.085 0.0135 .0043 3 (1) .1365 .066/.039 .1355/.1305 0.0135 .0025 3(2) .1365 .066/.039 .1347/.13 0.0135 .0025 4 .1365 .066/.039  .136/.132-0.0135 .00225  .133 5 .1365 .090/.065  .136/.133 0.0135 .006

Example Optical Micrographs of Stents

As indicated above, a polymeric stent with induced polymer chainalignment may be more resistant to cracking during use, includingcrimping, delivery, and deployment. FIGS. 17 and 18 depict opticalmicrographs of crimped stents made from a biodegradable polymer (100%poly(lactic acid) polymer). The stent shown in FIG. 18 was fabricatedfrom a tube radially deformed with a compressible fluid as depicted inFIGS. 5A-B. FIG. 17 illustrates cracks 850 in a stent with no inducedradial deformation. No cracks are observed in the stent depicted in FIG.18.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

1. A method for fabricating an implantable medical device having a finaldiameter, comprising: disposing in a cavity a polymer tube having aninitial diameter defined by an outer surface thereof, the cavity beingformed by sliding wedges having respective abutting surfaces defining anopening of the cavity, the sliding wedges configured for being displacedradially outward when the tube is radially expanded within the cavity;disposing the abutting surfaces of the sliding wedges at about the outersurface of the tube; and radially expanding the tube from the initialdiameter to the final diameter, wherein the sliding wedges apply aradial restraining force on the expanding tube as the outer surface ofthe tube contacts the abutting surfaces and displaces the sliding wedgesradially outward; and fabricating the implantable medical device fromthe expanded tube.
 2. The method of claim 1, wherein the polymercomprises a bioabsorbable polymer.
 3. The method of claim 1, wherein thetube is deformed axially and radially.
 4. The method of claim 1, whereinthe tube is radially expanded by increasing a pressure in the tube. 5.The method of claim 4, wherein the pressure is increased by conveying afluid into the tube.
 6. The method of claim 5, wherein the fluidcomprises an inert gas comprising air, oxygen, nitrogen, and/or argon.7. The method of claim 1, wherein a longitudinal axis of the cavity isparallel or approximately parallel to a cylindrical axis of the tube. 8.The method of claim 1, wherein the outer surface is exposed to at leastsix sliding wedges, each having an abutting surface and the respectiveat least six abutting surfaces are arranged to form a cavity thatapproximates a circle.
 9. The method of claim 1, wherein a rate ofdeformation of the tube is slower than a rate of unrestraineddeformation of the tube by the restraining force applied to the tube bythe sliding wedges.
 10. The method of claim 1, wherein a rate ofdeformation of the tube is controlled to increase a circumferentialstrength and circumferential modulus of the tube more than anunrestrained deformation of the tube.
 11. The method of claim 1, whereina rate of deformation is controlled to reduce or eliminate axialdeformation.
 12. The method of claim 1, wherein the radial expansion ofthe tube increases the radial strength and modulus of the tube.
 13. Themethod of claim 1, further comprising heating the tube prior to,contemporaneously with, and/or subsequent to radially expanding thetube.
 14. The method of claim 1, wherein the tube is heated by heatingthe abutting surfaces prior to radial expansion of the tube.
 15. Themethod of claim 1, wherein the tube is radially expanded at atemperature greater than or equal to a glass transition temperature ofthe polymer and less than or equal to a melting temperature of thepolymer.
 16. The method of claim 1, wherein the cavity is formed by aniris mechanism comprising the sliding wedges.